I have been doing some long overdue maintenance on my rolling stock. I have noticed wide variability in how well individual cars roll - some seem to go forever with a small push, others barely glide a few inches. Is there any standard for measuring this? For example, suppose an 8" piece of track is raised 1" at one end and the amount of glide on a level flex track is then measured. Or are there some rough rules of thumb on when one should seriously think about replacing the wheels, or trucks, or both? I would appreciate any comments or suggestions.